1. Field of the Invention
The invention relates to a phase demodulator, a phase difference detector and an interferometric system using the phase difference detector in which phase-modulated signals are transformed into amplitude-modulated signals for real-time measurement.
2. Description of the Related Art
A phase demodulator is used to demodulate phase-modulated signals, and is widely employed in the fields of communication, data transmission and interferometry. A conventional phase demodulator can include a phase-sensitive detector and a phase-locked loop for demodulation purposes. Alternatively, the output of a zero-crossing circuit for frequency-modulated signals can be fed to a counter so as to detect frequency counts (fs) for a test signal, from which the frequency counts (fr) for a reference signal is subtracted to obtain a frequency difference (Δf), i.e. Δf=fs−fr. Phase change is then detected with the use of an integrating circuit. Furthermore, a digital phase comparator can be employed to measure the phase difference between test and reference signals. A voltage signal is then outputted according to the magnitude of the output of the phase comparator. In the aforesaid conventional methods, phase difference, and thus the measured phase, is obtained by comparing test and reference signals using analog or digital means.
On the other hand, in the field of interferometry, phase changes are generally measured on the bases of light wavelength and optical interference in order to measure displacement, angle, length, vibration and other physical properties. Lasers are used as light sources for such interferometers. It is noted that changes in environmental conditions, such as temperature, can reduce the accuracy of phase measurement by optical interferometers. It has been proposed heretofore to construct optical interferometers with a common path configuration in order to minimize background noise due to environmental disturbance during operation thereof.